Mounting device for high frequency microwave devices

ABSTRACT

In a package mounting structure for mounting a package on a case, wherein the package internally incorporates at least one of a high-frequency transistor, MIC and MMIC used in the microwave to millimeter-wave band, and a base thereof is formed of metal and serves as ground, an electrically conductive sheet having excellent thermal conductivity and exhibiting restorability and having a size identical with that of the base of the package is laid on the case at a package-bearing location, the package and sheet are fastened together by two or more screws, and the sheet is mounted on the case while it is pressed by a pressing force of 10 N/cm 2  or greater owing to fastening.

BACKGROUND OF THE INVENTION

This invention relates to a mounting structure of a package, whichincorporates at least one of a high-frequency transformer, MIC(Microwave Integrated Circuit) and MMIC (Monolithic Microwave IntegratedCircuit) used in the microwave to millimeter-wave band, or of asuperconducting circuit board. More particularly, the invention relatesto a mounting structure whereby a thermally excellent state can beobtained without sacrificing the microwave characteristic.

Any of the following methods has been implemented heretofore in order tomount a microwave circuit, which handles signals in the microwave tomillimeter-wave band, on a case (heat sink): {circle around (1)} thecircuit is mounted upon interposing a soft metal such as indium betweenthe case and the package or board; {circle around (2)} the circuit ismounted upon applying a silicon compound or grease between the case andthe package or board; {circle around (3)} the circuit is mounteddirectly without doing any of these; and {circle around (4)} the circuitis mounted on the case directly by soldering.

Further, in the case of a low-frequency circuit, use has been made ofthe fact that a graphite sheet exhibits thermal anisotropy, wherein theheat that evolves from the heated portion of the low-frequency circuitis transported to a remote location by the graphite sheet so that theheat is dissipated. A further method that has been employed similarlymakes use of the properties of a graphite sheet, interposes the graphitesheet between the heat-producing portion of the package and the case andapplies pressure to the package as by auxiliary hardware to allow heatto escape. However, these expedients are applied to low-frequencycircuits and are not implemented in the case of microwave circuits.

Graphite is an allotrope of carbon and in one sense is akin to diamondbut in another is merely the lead of a pencil. A graphite sheet isobtained by molding graphite, which is close to diamond that has anattractive crystal alignment, into sheet form and exhibits a highthermal conductivity second to that of diamond. The thermal conductivityof copper or aluminum is less than one-half that of graphite in onedirection. For these reasons, a graphite sheet is ideal as a heattransfer material. However, since a graphite sheet is not in liquid formas in the manner of grease, making it adhere perfectly is a difficultchallenge.

Problems of Mounting Structures for Microwave and Millimeter-WaveCircuits

In the usual mounting structure of a microwave and millimeter-wave bandcircuit, a package internally incorporating a transistor, MIC or MMICthat evolves a high degree of heat is mounted on a metal case and acircuit board on which microstrips have been patterned is disposed aboutthe package. In order to facilitate the dissipation of heat produced bythe package, the area of contact between the package and the case hasits surface roughness removed to improve planarity and provide as muchsurface contact as possible.

In the microwave and millimeter-wave band, however, it is required thatthe lower part of the package be grounded in its entirety. No matter howmuch surface roughness and planarity are improved, contact is pointcontact in micro terms. In particular, if the package bends due to heat,ground will not be stable and good electrical characteristics (microwavecharacteristics) will not be obtained. Further, the heat-dissipatingeffect is limited, the package warps owing to evolution of heat andcontact worsens, thus further destabilizing ground. Though warppreventing screws or the like below the heat-producing body can help,the only method available is to screw down the periphery of the packagefrom above. Even if grounding is achieved, the temperature of thesemiconductor may exceed the absolute maximum rating if powerconsumption is high when one takes thermal resistance into account.

A method of securing the package to the case (heat sink) by solder isavailable as a method of solving the above-mentioned problem. If thepackage is secured by soldering, however, it cannot be moved again. If amalfunction or the like occurs, therefore, it is necessary to replacethe case in its entirety and not just the package in which thetransistor, MIC or MMIC is mounted. This means that all of the circuitsincluded in the case must be replaced. The problem that arises is anincrease in cost.

Further, there is a method that uses a silicon compound as a materialthat may be replaced subsequently. However, since the compound is aninsulator, a problem encountered is that grounding is difficult toachieve. In addition, the technique for applying the compound thinly andskillfully is difficult. This is a task requiring skill.

A method of laying an indium sheet is available as a method of achievingground. However, the sheet is squashed by pressure from above and has noresiliency. If it is used for a long period of time, therefore, a gapmay form between the package and the case. This gives rise toreliability-related problems and a change in electrical characteristics.

Further, in the case of a low-frequency circuit, sandwiching a graphitesheet between the heat-producing portion and the case has beenconsidered. A structure of the kind illustrated in the sectional view ofFIG. 20 has been proposed. Specifically, a cap filter 3 is interposed ina gap between a heat source (MPU, PA, graphic chip, etc.) 2 provided ina package 1 and the upper part of the package, a graphite sheet 4 islaid on the top surface of the package and a case (heat sink) is laid onthe graphite sheet and fixed, thereby achieving mounting. If thisexpedient is adopted, heat is transferred in the directions indicated bythe arrows. If the graphite sheet 4 is used, a non-uniform heatdistribution HTD1 that prevails when the graphite sheet is not usedchanges to a heat distribution HTD2 owing to dissipation of heat throughthe entirety of the heat sink and the heat at the heat-producing portionis dissipated. FIG. 21 is a diagram for describing a DVD-RAM mountingstructure. A pick-up 8 is provided below a printed board 7 on which CPUs6 a, 6 b are mounted, and a PGS graphite sheet 4 is placed on aheat-producing portion such as a light-emitting diode of the pick-up 8.In the arrangement of FIG. 21, heat evolved by the CPUs and pick-up istransferred to and dissipated from an external case 9 via the graphitesheet 4.

However, it is difficult to apply the mounting structures of FIGS. 20and 21 to mounting of a microwave circuit. The reason is that in thecase of a microwave circuit, it is necessary to place a circuit board,the principle part of which is a microstrip, in a surrounding area inclose proximity to the package (transistor, MIC or MMIC).

In the case of a low-frequency circuit, as shown in FIG. 21, no problemsarise even if connection is by point contact. However, in the case of apackage internally accommodating a microwave circuit, a surface-contactground structure becomes necessary. In the mounting arrangements ofFIGS. 20 and 21, the graphite sheet exhibits superior heat transfer inthe transverse direction, as indicated by the arrows. However, since asingle sheet is thin, there is little spread of heat in the transversedirection, as indicated by these drawings.

Further, the structure in FIG. 20 is such that in order to press theheat-producing portion against the graphite sheet 4, it is held downfrom above by the case 5. In this case, the space above the surroundingcircuit also is narrowed. In the case of a microwave circuit, the spaceabove the peripheral circuitry is reduced. As a result, even ifperipheral circuitry is installed, ground above the circuit is too closeand it is difficult to obtain the usual microwave characteristics.Further, in order to dissipate heat in practical terms, special hardwarefor strong hold-down is required.

Though the attaching method of the case is not clarified in FIG. 21,fixing the CPU package to the heat dissipating portion (the externalcase) by the graphite sheet 4, which is an adhesive layer, iscontemplated. However, the existence of an adhesive layer enlargesthermal resistance. Further, the lower part of the package of thetransistor, MIC or MMIC used in a microwave and millimeter-wave bandpower circuit, which evolves a large amount of heat, is greater than100° C. Since an adhesive layer can withstand a high temperature only onthe order of 100° C., the sheet constituting the adhesive layer cannotbe used in mounting a microwave circuit in the manner of the mountingstructure of FIG. 21. Further, since the mounting structure of FIG. 21is for dealing with a low-frequency circuit, the special nature ofmicrowave and millimeter-wave high-frequency circuits is not taken intoconsideration and there are limitations with regard to the mounting of amicrowave circuit that requires the placement of a board, the principlepart of which is a microstrip, in a surrounding area in close proximityto the package (transistor, MIC or MMIC).

In FIG. 22, a diagram in which pressure is applied to the package 1 isdrawn in order to measure thermal resistance, and the entirety of thepackage 1 is subjected to pressure. This also is a structure in whichthe package is pressed from above. Problems the same as those of FIGS.20 and 21 occur. That is, with a structure in which the entirety of thepackage of FIG. 22 is pressed down from above, the space above thesurrounding circuitry takes on the same height as that of the package.In the case of a microwave circuit, the space above the circuitdiminishes. As a result, even if circuitry is installed, ground abovethe circuit is too close and it is difficult to obtain the usualmicrowave and millimeter-wave characteristics. Furthermore, in a casewhere pressure is applied from above, there are instances where Teflonor the like having a low dielectric constant is used. However, thismerely gives an assist in microwave and millimeter-wave applications andthe package of a transistor, MIC or MMIC cannot be secured by pressurealone.

With regard to dissipation of heat, configurations in which the heat isbrought from the heat-producing portion to the heat-dissipating portionby utilizing a graphite sheet have been considered, as seen in theJapanese Patent Application Laid-Open Nos. 8-23183, 11-110084 andJapanese Patent Application Nos. 11-149329, 10-3333202, 11-128567 and10-51170. However, these examples of the prior art give no considerationto the problem of ground, which is specific to microwaves. Further,though the graphite sheet has a high thermal conductivity in thetransverse direction and therefore has some degree of effectiveness, thesheet has little thickness and exhibits only a small amount of heattransport. For example, if the amount of heat evolved by a transistor,MIC or MMIC is greater than 100 W, it is difficult to lower the channeltemperature of the transistor by more than 10° C. Further, in the caseof a microwave or millimeter-wave circuit, the circuit board is verynearby and therefore it is necessary to transport heat while avoidingthe circuit board. In other words, heat can only be transported fromportions not related to the RF characteristic, in which case the amountof heat transported is small and satisfactory effects are not obtained.

For example, Japanese Patent Application Laid-Open No. 8-21183 disclosesa cooling structure for cooling a heated member in a low-frequencycircuit, i.e., a cooling structure for a heat-producing membercharacterized by inclusion of a heat-dissipating part made of graphiteexhibiting a high degree of alignment. It is indicated that theheat-dissipating part in this cooling structure may be {circle around(1)} a heat sink for cooling an electronic part that is a memberevolving heat, {circle around (2)} a sealing member for sealing anelectronic part that is a member evolving heat, or {circle around (3)} aheat-dissipating member connecting an electronic part that is a memberevolving heat and a heat-dissipating body for dissipating heat.

According to method {circle around (1)}, the heat-dissipatingcharacteristic of the heat sink is excellent. However, in a case wherethe heat-producing body is placed inside a package, as in a microwave ormillimeter-wave circuit, a problem which arises is that a satisfactoryheat-dissipating effect is not obtained unless there is sufficientthermal transfer from the package to the heat sink. Further, accordingto method {circle around (2)}, it is required that the sheet have anadhesive function. The graphite sheet described, however, does notillustrate an adhesive function. If an adhesive is used, the thermalconductivity will decline owing to the adhesive and the heat-dissipatingeffect will be degraded. According to method {circle around (3)}, theconnecting member has a shape of the kind shown in FIG. 1 of JapanesePatent Application Laid-Open No. 8-21183. However, owing to the smallthickness of the sheet, the amount of heat transported is small. Forexample, if the amount of heat evolved by a transistor, MIC or MMIC is100 W, it is difficult to lower the channel temperature of thetransistor by more than 10° C.

Further, as illustrated in Japanese Patent Application Laid-Open No.10-283650, there is an example in which a heat sink is fixed using aconnecting member in a laser-beam generating apparatus. Though circuitdevices can be used independently in a case where there is no relationto frequency, as in this example, the microwave circuit is such thatanother microwave circuit is in close proximity to the surroundings.This arrangement is not practical. Further, with the method using aconnecting member, a problem which arises is that a new member referredto as a connecting member is required. That is, in a case where thepackage is fixed, the connecting-member material and connectingconditions are required and a problem which arises is an increase inmember other than the usual screws for fastening the package.

A structure that allows heat to escape using an indium sheet isillustrated in Japanese Patent Application Laid-Open No. 5-75283.However, indium has almost no restoring force, undergoes deformationwith use over an extended period of time, a gap is produced betweenitself and the case and the heated body may peel off. Problems arise interms of thermal conductivity and maintenance of ground.

The foregoing is prior art for dissipating heat that has evolved fromthe heated body. With a high-frequency circuit or high-speed digitalcircuit that uses superconductivity for dealing with microwave andmillimeter-wave high-frequency electromagnetic components, it isrequired that members be kept at extremely low temperatures. FIGS. 23Aand 23B are diagrams for describing a cooling structure. FIG. 23A is aperspective view in which an upper cover 10 of a high-frequency circuitdevice employing superconductivity has been removed, and FIG. 23B is asectional view taken along line A′A of the upper cover. A base metalboard (Invar, Kovar, copper) 13 on which a superconducting RF circuitboard 12 has been mounted is secured to a case 11 by board-mountingscrews 14 a to 14 f. Indium sheets 15 a, 15 b for removing heat are laidbetween the case 11 and base metal board 13 and between thesuperconducting RF circuit board 12 and the base metal board 13.Furthermore, the case 11 is secured on a cold head 16 by screws 17 a, 17b via an indium sheet 15 c. The arrangement is such that a coolant suchas LN or LHe is passed through the interior of the cooling end 16 orsuch that cooled He gas fed from a refigerator (not shown) is passedthrough the cooling end. At an operating temperature of about 4K thatadopts liquefied He (LHe) as the coolant, a superconducting RF circuitthat uses an Nb superconducting film or YBCO, BSCCO film can be cooled.At an operating temperature of about 77K that adopts liquefied nitrogenas the coolant, or at 50 to 80 K using a refigerator, a superconductingRF circuit that uses YBCO or BSCCO film can be cooled. In FIG. 23A,reference numeral 18 denotes a superconducting film pattern on thesuperconducting RF circuit board 12, reference numerals 19 a, 19 bdenote coaxial connectors, and reference numerals 20 to 20 d denoteholes for receiving screws.

The evolution of heat by a circuit in a portion to be cooled is smallerby an order of magnitude in comparison with ordinary semiconductorcircuits and wiring. However, heat that penetrates from the exterior ofthe cooling device penetrates the portion to be cooled (thesuperconducting RF circuit board 12) through the case 11. The rise intemperature owing to penetration of heat is lowered by the cold head 16.However, the indium sheets 15 a to 15 c exhibit almost no restoringforce and have adhesion to with respect to many case materials andboards. When the case 11 is removed, therefore, the indium sheet 15 cundergoes shape deformation and re-use in the same shape is difficult.In case of re-use, a gap tends to form between the sheet and case,temperature unevenness develops within the board and often it isdifficult to achieve active operation and transmission-circuit operationby superconductivity with a high degree of reproducibility.

Further, in the cooling structure of FIGS. 23A, 23B, problems are EMC,vibration in the internal circuitry and degradation of characteristics.The case 11 of the microwave or millimeter-wave circuit usually has thecover 10 and a bottom cover fixed to it by screws 17 a to 17 b. Since agap is formed in such case, radio waves leak from the gap, therebybringing about the problems EMC, vibration in the internal circuitry anddegradation of characteristics. EMC involves two phenomena, namelyleakage of radio waves to the outside and penetration of external radiowaves. Vibration is caused by part of the output being fed back to theinput through the gap. Further, the degradation of characteristics alsois caused by coupling between circuits through the gap. For example,this leads to a frequency characteristic exhibiting a break.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a mountingstructure for efficiently dissipating heat evolved by a packageinternally incorporating at least one of a high-frequency transistor,MIC and MMCI used in the microwave band or millimeter-wave band, therebymaking it possible to readily obtain a thermally excellent state.

Another object of the present invention is to provide a mountingstructure for mounting a superconducting circuit board and a graphitesheet that require thermal uniformity on a case by fastening themtogether with screws, thereby making it possible to readily obtain athermally excellent state without sacrificing microwave characteristics.

A first aspect of the present invention is a package mounting structurefor mounting a package on a case, the package internally incorporatingat least one of a high-frequency transistor, MIC and MMIC used in themicrowave to millimeter-wave band, a base thereof being formed of metaland serving as ground.

In the first mounting structure, an electrically conductive sheet, e.g.,a graphite sheet, having excellent thermal conductivity and exhibitingrestorability and having a size identical with that of the base of thepackage is laid on the case at a package-bearing location, and thepackage and sheet are fastened together and attached to the case by twoor more screws. In this case, the package is provided with fully open orhalf-open holes, the case is provided with screw holes, portions throughwhich the screws are passed are made sheetless, and by fastening thepackage and sheet together by screws, the sheet is mounted on the casewhile the sheet is pressed by a pressing force of 10 N/cm² or greater.In accordance with the first mounting structure, a rise in thetemperature of the package can be suppressed through a simple method, athermally excellent state can be obtained and electrical characteristicssuch as power application efficiency can be improved.

In a second mounting structure, in a case where steps are presentbetween a package-bearing location and circuit-board-bearing location onthe case, step portions, which are among step portions on four sides ofthe package-bearing location and through which microwaves do not pass,are inclined, an electrically conductive sheet having excellent thermalconductivity and exhibiting restorability is laid on the case on theentirety of the package- and circuit-board-bearing locations, and eachof the package and circuit board and the sheet are fastened together andattached to the case by two or more screws. In this case, holes areprovided in the sheet at portions through which the screws are passed,thereby resulting in a sheetless state, and by fastening the package andsheet together by screws, the sheet is mounted on the case while thesheet is pressed by a pressing force of 10 N/cm² or greater. Inaccordance with the second mounting structure, besides effects similarto those of the first mounting structure, the sheet is prevented frombeing severed by the inclined step portion and grounding of the entiresheet can be maintained.

A second aspect of the present invention is a mounting structure formounting a superconducting circuit board in a metal case. In thismounting structure, an electrically conductive sheet, e.g., a graphitesheet, having excellent thermal conductivity and exhibitingrestorability is laid on the bottom of a box-shaped metal case, asuperconducting circuit board is placed on the sheet, and thesuperconducting circuit board and sheet are fastened together andattached to the metal case by two or more screws. In this case, holesare provided in the sheet at portions through which the screws arepassed, thereby resulting in a sheetless state, and by fastening thepackage and sheet together by the screws, the sheet board is mounted onthe case while the sheet is pressed by a pressing force of 10 N/cm² orgreater. Further, another electrically conductive sheet exhibitingrestorability and provided with holes at portions through which screwsare passed is placed on peripheral top portions of the box-shaped metalcase, a cover of the case is placed thereon, and the cover and sheet arefastened together by screws to seal the interior of the case. Further, arecess (groove) smaller than the thickness of the sheet is formed ineither the peripheral top portion or cover of the case to therebystrengthen the degree of sealing. In accordance with this mountingstructure, the superconducting circuit board, which requires thermaluniformity, and the graphite sheet are fastened together by screws tomount the superconducting circuit board on the case, thereby making itpossible to readily obtain a thermally excellent state withoutsacrificing microwave characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of a mounting method for fastening apackage and a sheet together and mounting them on a case by a singlescrew;

FIG. 2 is an exploded perspective view illustrating a package mountingstructure according to a first embodiment;

FIG. 3 is a sectional view in case of sectioning along signal direction;

FIG. 4 is a diagram for describing sizes of a package and graphitesheet;

FIG. 5 is a diagram for describing holes in a package and graphitesheet;

FIG. 6 is a diagram showing the relationship between contact pressure ofa package and temperature rise of the package for the purpose ofdeciding pressing force;

FIG. 7 is a sectional view of a modification in which the size of asheet is made smaller than the base size of a package and the same asthe size of a heat-producing portion of the package;

FIG. 8 is a diagram for describing sizes of a package and graphite sheetin a modification;

FIG. 9 is a diagram illustrating the relationship between amount ofevolved heat and temperature rise of a package;

FIG. 10 is a microwave characteristic diagram in cases where a sheet isand is not present;

FIG. 11 is a diagram illustrating the relationship between amount ofevolved heat and temperature rise when area of the sheet is changed;

FIG. 12 is a diagram showing a comparison of temperature rise silicongrease and of a sheet;

FIG. 13 is a diagram for describing effects of a sheet when the mountingsurface is roughened;

FIG. 14 is an exploded perspective view illustrating a package mountingstructure according to a second embodiment;

FIGS. 15A, 15B are sectional views of a package mounting structure;

FIG. 16 is an exploded perspective view illustrating a package mountingstructure according to a third embodiment;

FIGS. 17A, 17B are sectional views of a package mounting structureaccording to the third embodiment;

FIGS. 18A, 18B are diagrams useful in describing a mounting structurefor mounting a superconducting circuit board on a metal case;

FIG. 19 is diagram useful in describing another mounting structure formounting a superconducting circuit board on a metal case;

FIG. 20 is a first explanatory view of a mounting method using agraphite sheet according to the prior art;

FIG. 21 is a second explanatory view of a mounting method using agraphite sheet according to the prior art;

FIG. 22 is a third explanatory view of a mounting method using agraphite sheet according to the prior art; and

FIGS. 23A, 23B are diagrams for describing a cooling structure accordingto the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (A) Principles of the Invention

It is important to so arrange it that heat produced by a package can bedissipated efficiently with almost no modification in the mounting ofordinary microwave to millimeter-wave circuits. To accomplish this, asheet should be as follows:

{circle around (1)} it should have excellent electrical conductivitywithout any change in microwave characteristics;

{circle around (2)} it should have a restoration force in the manner ofa cushion and should maintain the restoration force for a long period oftime; and

{circle around (3)} it should be electrically conductive in air and bemountable on a case when fastened together with a package by screws.

Furthermore, if a sheet-like member is used, it is not in liquefied formin the manner of grease and therefore mounting is simplified.

A graphite-sheet structure is available as a sheet having theabove-mentioned properties, and the method of manufacture is known from,e.g., Japanese Patent Application Laid-Open No. 10-345665. Further,though a graphite sheet is ideal as the sheet, a second-best sheet is anSiC substrate, etc., on which a carbon nanotube film (having a thicknessof 200 nm or greater) has been formed. This also is effective because ithas properties similar to those of the graphite sheet in that the carbonnanotube film portion has restorability. Further, with a package or thelike for microwave to millimeter-wave circuits, the package serves asground. A package with little electrical conductivity is a problembecause satisfactory grounding cannot be achieved. However, theabove-mentioned graphite sheet and SiC substrate on which the carbonnanotube film has been formed have excellent electrical conductivity andexhibit no degradation in microwave characteristics ascribable to poorgrounding.

When the package and sheet are fastened together and mounted on the caseby screws, it is required that mounting be performed while subjectingthe sheet to considerable pressing force, e.g., a pressing force of 10N/cm² or greater. This is to bring the sheet and the package as well asthe sheet and case into intimate contact (surface contact). By virtue ofsurface contact, the efficiency of thermal conduction is improved andsatisfactory grounding is achieved, thereby making it possible toprevent degradation of microwave characteristics.

The fastening together of the package and sheet by screws is applicableto the package of a comparatively small (50 to 300 mm² at most)microwave circuit. The reason for this is that the package of themicrowave circuit is usually secured by screws. By using screws, thesheet can be subjected to a pressing force greater than that achieved bysecuring with pins. Further, with use of a single screw 21, as shown inFIG. 1, the pressure at the periphery at point B devoid of a screw issmall and therefore not enough force is obtained to transfer heat. Forthis reason, two or more screws are required. In FIG. 1, referencenumeral 22 denotes the surface of a case, 23 a thermally conductivesheet, 24 a package and 25 hole into which the screw is inserted.

By virtue of the method of fastening the package and sheet together bytwo or more screws and mounting them on the case while pressing thesheet with a pressing force of 10 N/cm² or greater, the problem setforth in Japanese Patent Application Laid-Open No. 8-23183 alreadydescribed can be solved and so can the problem of poor thermalconductivity owing to weak pressure or adhesive material illustrated inFIG. 21. Further, it is unnecessary to restrain the sheet by screws fromabove using a special jig.

Since the sheet is thin, there is little effect even if heat istransferred from the heat-producing body to the heat sink by making aconnection to the heat sink, as in Japanese Patent Application Laid-OpenNo. 8-23183. However, by laying the sheet directly beneath theheat-producing body of the package, fastening the sheet and packagetogether by screws and applying a pressing force of 10 N/cm² or greaterto the sheet to achieve mounting on the case, it is possible to transferheat to the area directly underlying the pressing force and suppress arise in the temperature of the package.

Here it is necessary that the signal propagation portion extend as closeas possible to the position of ground in the microwave ormillimeter-wave circuit. It is required, therefore, that the size of thesheet be the same as that of the package (i.e., that the overall sheetbe ground). Further, performance will be unaffected even if the sizes ofthe package and sheet differ somewhat in a direction in which there isno microwave or millimeter-wave signal, i.e., in a direction at rightangles to the direction in which microwaves propagate.

(B) First Embodiment

FIG. 2 is an exploded perspective view illustrating a package mountingstructure according to a first embodiment, FIG. 3 a sectional view incase of sectioning along signal direction, and FIG. 4 a diagram fordescribing sizes of a package and graphite sheet. Circuit boards 52, 53on which microstrips have been patterned are placed on a case (heatsink) 51, which serves as a heat-dissipating member, so as to sandwich apackage 54 between them. Further, screw holes 51 a, 51 b for attachingthe package are formed in the case 51.

The package 54 internally incorporates at least one of a high-frequencytransistor, MIC and MMIC used in the microwave to millimeter-wave band.The package has the external shape of a projection and the base portionthereof is formed at both ends to have holes 54 a, 54 b through whichscrews 56 a, 56 b for attaching the package are inserted. Further,packet input/output terminals 54 c, 54 d (not shown in FIG. 2; refer toFIG. 3) are formed on both sides of the protruding portion of thepackage. The sheet 55 has a size the same as that of the base of package54 (see FIG. 4) and the portions through which the screws 56 a, 56 b arepassed are opened to form holes 55 a, 55 b, whereby a sheetless state isobtained. The sheet 55 is required to have a restorability of 20% orgreater, good thermal conductivity and electrical conductivity. Anexample is a graphite sheet.

To attach the package to the case, the graphite sheet 55 is laid on thecase 51 on the package-bearing location in such a manner that the holes55 a, 55 b will align with the screw holes 51 a, 51 b, then the package54 is placed thereon in such a manner that the holes 54 a, 54 b willalign with the holes 55 a, 55 b, and two or more (two in theillustration) screws 56 a, 56 b are inserted into the respective holesand the package 54 and sheet 55 are fastened together and attached tothe case 51. In this case, it is required that mounting on the case 51be performed while pressing the sheet 55 with a prescribed pressingforce by tightening the screws. Finally, the microstrips on the circuitboards 52, 53 and the input/output terminals 54 c, 54 d (see FIG. 3) ofthe package are connected by solder 56. It should be noted that theholes 54 a, 54 b and 55 a, 55 b provided in the package 54 and sheet 55can be half-open holes, as shown in FIG. 5.

FIG. 6 is a diagram showing the relationship between contact pressure ofthe package and temperature rise of the package for the purpose ofdeciding pressing force. Cases where amount of heat evolved is 60 W(curve A) and 90 W (curve B) are illustrated. The horizontal axis is aplot of contact pressure (N/cm²) and the vertical axis a plot of packagetemperature rise. The rate of temperature rise differs about a boundaryof 5 to 10 (N/cm²) irrespective of the type of sheet or amount of heatevolved. It will be understood from this characteristic diagram that therise in the temperature of the package can be suppressed if a contactpressure of 10 (N/cm²) or greater is applied.

FIG. 7 is a sectional view of a modification in which the size of thesheet 55 is made smaller than the base size of the package and the sameas the size of a heat-producing portion of the package, and FIG. 8 is adiagram for describing sizes of the package and graphite sheet in themodification. Portions identical with those shown in FIGS. 3 and 4 aredesignated by like reference characters.

The state of ground in the first embodiment and modification will beunderstood from FIGS. 3 and 7. Specifically, in the first embodiment,grounding of the package 54 is achieved satisfactorily from the entiretyof its base surface because the sheet 55 has an electrical conductivityequivalent to that of metal. In the modification, however, part of thebase is not grounded. Thermally speaking, the first embodiment of FIG. 3and the modification of FIG. 7 are almost same. With the modification ofFIG. 7, however, a problem which arises is that part of the base isremote from ground (the case) and the microwave characteristic isdisturbed as a result.

When long-term reliability is considered, it is required that the sheet55 exhibit a recovery force (restorability of 20% or greater). Thereason is that the recovery force allows grounding, which is innatelyunstable, to be obtained and enhances the heat-escape effect. Thiseffect can be attained by the simple method of jointly fastening thepackage 54 by the screws 56 a, 56 b and merely adopting a sheet 55 of ahigher price. Here a configuration in which the screw holes throughwhich the screws are passed are provided in the sheet is desirable.

The amount of heat evolved by accommodating a FET amplifier of a singlestage in the package 54 was investigated. In FIG. 2, tightening torqueof the two M2.3 screws 56 a, 56 b is 3.5 kgf. The area below the package54 is 3.5×1.5 cm=5.25 cm². The sheet 55 used had holes provided in thisportion in order to pass screws therethrough. The amount of heat evolvedin this case and the value of rise in temperature on the top side of thepackage are illustrated in FIG. 9. Here reference character A representsa characteristic in the absence of the sheet, and B a characteristicwhen the sheet is present. It will be appreciated that thecharacteristic B, which is obtained when the sheet is interposed, isexcellent as it is thermally more than double the characteristic A inthe absence of the sheet.

A problem arises if the microwave characteristic changes. FIG. 10illustrates the result of experiments for evaluating the microwavecharacteristic. Here character A represents output voltage when thesheet is present, B the output power in the absence of the sheet, A′ thepower application efficiency when the sheet is present and B′ the powerapplication efficiency in the absence of the sheet. Excellent values areobtained for both the input/output characteristic and efficiencycharacteristic when the sheet is present. The reason for this is thatthe electrical conductivity of the sheet is equivalent to that of metaland therefore is excellent, as a result of which the package is groundedover its entire surface. It will be understood that the microwavecharacteristic is not affected.

Since the sheet 55 is thin, however, there is little effectiveness interms of allowing heat to spread transversely. For this reason, dataconcerning a rise in temperature in a case where the area of the sheet55 is changed was collected. However, the rate of rise in temperaturewas almost the same, as illustrated in FIG. 11. In particular, withsheet A, the rise in temperature was the same even in a case where thearea of the package was made greater than 3.5×1.5 cm. It will beunderstood from this data also that placing the sheet below theheat-producing body is effective and that a heat-dissipating member incontact with a heat-producing body does not provide much effect.

The values of a rise in package temperature versus amount of heatevolved in a case where the package is embraced by silicon grease(KE1223) and the sheet are plotted in FIG. 12. Reference character Arepresents the characteristic obtained when silicon grease is interposedand B the characteristic obtained when the sheet is interposed. In termsof the thermal characteristic, it will be understood that the rise intemperature is lower for the sheet to the extent that it has the betterthermal conductivity. Since the electrical characteristic cannot bemeasured in the case of an insulator such as silicon grease, it is notsuited to a microwave circuit.

FIG. 13 is a characteristic diagram of amount of heat evolved andtemperature rise when the surface of the case has been roughened. Herereference character A represents a characteristic in a case where thereis no sheet and the surface of the case has not been roughened, B acharacteristic in a case where there is no sheet and the surface of thecase has been roughened, C a characteristic in a case where a sheet ispresent and the surface of the case has been roughened and D acharacteristic in a case where a sheet is present and the surface of thecase has not been roughened. In the absence of the sheet, temperaturerose greatly from 32.5° C. to 54.1% at 80 W of evolved heat, or by 166%.When the sheet was interposed and pressure applied thereto by twoscrews, the rise in temperature was from 13.8° C. to 14.9° C., or a lowrate of increase of 108%. It will be understood from the characteristicdiagrams of FIGS. 12 and 13 that there is sufficiently close contactbetween the sheet and case and that the method of the first embodimentis effective.

In accordance with the first embodiment, a sheet having an electricalconductivity equivalent to that of a metal and exhibiting a certaindegree of restorability is in contact with the entire surface of thebase of a package in a microwave circuit that uses the package. Stablegrounding is achieved as a result. Further, since the sheet is insurface contact even in a case where an element evolves heat, pathsalong which heat flows from a large number of points of point contactare produced and the temperature of the heat-producing element islowered. Further, the sheet is interposed between the package and caseand is secured by a plurality of screws. The sheet can be brought intocontact with the package and case while under pressure. Moreover, sincethe sheet has restorability, it can retain its shape over an extendedperiod of time. Further, only the cost of the sheet rises and there isno such increase in the case of the other components. Hence theinvention can be worked without additional cost. Further, sinceretention is by screws, the position of ground on the upper side is notaffected by the height of the package. Further, in accordance with thefirst embodiment, the channel temperature of the transistor, forexample, declines by several tens of degrees centigrade. As a result,gain, saturation output, efficiency and distortion characteristic areimproved.

(C) Second Embodiment

In the first embodiment of FIG. 2, the case 51 is not formed to have apackage accommodating portion and therefore positioning the sheet 55 andpackage 54 when the package is mounted is difficult. FIG. 14 is anexploded perspective view illustrating a package mounting structureaccording to a second embodiment, FIG. 15A is a sectional view takenalong line AA′ when sectioning is performed along the signal direction,and FIG. 15B is a sectional view taken along line BB′ when sectioning isperformed at right angles to the signal direction. Portions identicalwith those shown in FIGS. 2 and 3 are designated by like referencecharacters. This embodiment differs from the first embodiment in thatthe case 51 is formed to have a package accommodating portion 60.Providing the package accommodating portion 60 makes it possible toposition the sheet 55 and package 54 in simple fashion merely by placingthem in the accommodating portion 60 when the package is mounted.Furthermore, reference number 62 denotes a metal cover, 63 a 50-Ω lineand 64 the cross section of a ceramic portion.

(D) Third Embodiment

In the second embodiment, no problems whatsoever arise since a sheethaving a size the same as that of the package base need only be set inthe accommodating portion 60. However, there are cases where the sheet55 is laid integrally on the case on the entirety of the locations thatbear the package 54 and circuit boards 52, 53, and the package andcircuit boards are grounded by the sheet. In such cases, right-anglesteps 61 exist at four sides between the location that bears the package54 and the locations that bear the circuit boards 52, 53, as evidentfrom FIGS. 15A, 15B. Owing to the right-angle steps, the thin sheet 55is severed by the right-angle portions of the steps 61 when the circuitboards 52, 53 are installed following the laying of the sheet and thenthe package 54 is set upon the sheet and is fastened by the screws 56 a,56 b. Accordingly, in the third embodiment, if steps 61 exist betweenthe package-bearing location and circuit-board-bearing location on thecase 51, the step portions through which microwaves do not pass, whichstep portions are among the step portions on the four sides of thepackage-bearing location, are inclined.

FIG. 16 is an exploded perspective view illustrating a package mountingstructure according to the third embodiment. FIG. 17A is a sectionalview taken along line AA′ when sectioning is performed along the signaldirection, and FIG. 17B is a sectional view taken along line BB′ whensectioning is performed at right angles to the signal direction.Portions identical with those shown in FIGS. 14, 15A, 15B are designatedby like reference characters.

This embodiment differs from the second embodiment in that step portionsthrough which microwaves do not pass, which step portions are among thestep portions on the four sides of the accommodating portion 60, areinclined to form slanted step portions 62. If slanting is performed inthis manner, the sheet will not be severed at the step portions. Morespecifically, the sheet will not be severed by the slanted step portions62 even when the electrically conductive sheet having restorability islaid on the case on the entirety of the package- andcircuit-board-bearing locations, the circuit boards 52, 53 are installedon the sheet, the package 55 is subsequently set upon the sheet and bothare fastened together by the screws 56 a, 56 b. As a result, groundpotential of the entire sheet can be maintained and the package andcircuit boards can be provided with ground. It should be noted thatalthough the illustrated sheet has been provided with incisions at stepportions other than the slanted step portions 62, incisions are notnecessarily required. The entirety of the circuitry can be constructedby a single sheet. This is an effective method particularly when aplurality of transistors MICs and MMICs are installed.

(E) Fourth Embodiment

FIGS. 18A, 18B and FIG. 19 illustrate a fourth embodiment of a mountingstructure for mounting a superconducting circuit board on a metal case.FIG. 18A is a perspective view in which an upper cover has been removed,and FIG. 18B is a sectional view along line AA′ in which the upper coverhas been attached. As illustrated in the drawings, a sheet, e.g., agraphite sheet 73, having excellent thermal conductivity and exhibitingrestorability is laid on a bottom of a box-shaped case 71, asuperconducting circuit board 74 on which a superconducting RF circuitor the like has been mounted is placed upon the sheet, thesuperconducting circuit board 74 and sheet 73 are fastened together bytwo or more screws 75 a to 75 e, and the sheet 73 is attached to themetal case 71 while being pressed by a prescribed pressing force. Thepressing force is 10 N/cm², as in the first embodiment, and the sheet 73is mounted on the case 71 while being pressed by this pressing force.The size of the sheet is the same as that of the circuit board. Agraphite sheet 73′ is laid on a cold head 76, the case 71 is placed uponthis sheet and the sheet is fixed by four screws 77 a, 77 c, . . . . Thearrangement is such that a coolant such as LN or LHe is passed throughthe interior of the cold head 76 or such that cooled He gas fed from arefigerator (not shown) is passed through the cold head. In theillustration, reference numeral 78 denotes a superconducting filmpattern on the superconducting RV circuit board 74, 79 a coaxialconductor and 80 a to 80 d holes into which screws are inserted.

By thus providing the graphite sheet 73 and mounting the sheet 73 on thecase 71 while pressing the sheet 73 by a pressing force of 10 N/cm² orgreater, a thermally excellent state can be obtained without sacrificingthe microwave characteristics.

Further, by adding on the sheet 73′ in order to improve the thermalcontact between the case 71 and the cooling end 76, the coolingperformance can be improved further and a thermally excellent state canbe obtained. In this case, the sheet 731 is given a structure in whichit is provided with holes or partially cut away so as to avoid thescrews.

(F) Fifth Embodiment

A fifth embodiment is a mounting structure for solving the problems ofEMC, vibration of the internal circuits or and degradation ofcharacteristics. Specifically, as shown in FIG. 19, an electricallyconductive sheet (e.g. a graphite sheet) 91 having restorability andprovided with holes at portions through which screws are passed isplaced on peripheral top portions 71 a to 71 c of the box-shaped metalcase 71, a cover 70 of the case 71 is placed thereon and the cover 70and sheet 91 are fastened together by a plurality of screws 92.

Though the above-described sheet mounting method is easy, a problemwhich arises is that the sheet 91 shifts. Accordingly, a recess (groove)is formed in one of the case 71 or cover 70 (the top portions of thecase in the illustration) and the sheet 91 is placed in the recess,whereby it can be mounted without shifting. At this time it is necessaryto engineer contact between the cover 70 and case 71. In the case 71, agroove depth that is less than 80% of the thickness of the sheet 91 isappropriate. Further, the case 71 has the shape of a small box in orderto avoid vibration and a break in the characteristic. The groove on thecase side has its inner side cut away at the top portions 71 a, 71 b ofthe peripheral portions. The groove is cut into a U-shaped configurationat the top portion 71 c, which is other than the peripheral portions.The cut-away portion serves as a guide and makes it possible to easilymount the sheet 91. Moreover, sealing is possible.

Further, it is possible to obtain the pressing force applied to thesheet 91 by pressing the sheet against the case 71 by a plurality ofscrews 92 (six in the illustration) via the cover 70. Since gaps areclosed by adopting this expedient, radio waves will not leak from suchgaps and external radio waves can be prevented from penetrating into theinterior. As a result, the problems of EMC, vibration of internalcircuitry and degradation of characteristics can be solved. That is,vibration in the internal circuitry is caused by part of the outputbeing fed back to the input through the gap. Vibration does not occurbecause this cause is eliminated. Further, the degradation ofcharacteristics also is caused by coupling between circuits through thegap. By eliminating the gaps, degradation of characteristics, e.g., afrequency characteristic having a break, does not occur.

Holes HL are provided in the portions of the sheet 91 through whichscrews are passed. Further, this example is one in which the case sideis cut away. However, the effects are the same if the cover side is cutaway.

Since the sheet 73, which has electrical conductivity equivalent to thatof metal in terms of superconductivity and exhibits restorability, is incontact with the superconducting circuit board 74, the circuit board andcase are grounded in excellent fashion and the characteristics arestabilized. Further, since thermal conductivity of the sheet 73 in thetransverse direction is excellent, the superconductor is cooleduniformly and the characteristics are stabilized.

In accordance with the present invention, the rise in temperature of apacket is lowered from the conventional 35° C. to 12° C. in a case wherethe package houses a heat-producing body that gives off 80 W of heat, byway of example. As a result, in an example with a single amplifierstage, gain and output are improved by about 1.5 dB, efficiency isimproved by about 7 to 8%, and distortion becomes an excellent 4 dB.Further, the invention is effective even at cryogenic temperatures usedin superconductivity.

In accordance with the present invention, the sheet has electricalconductivity, unlike grease, and therefore it is possible to ground amicrowave package or board. Further, application unevenness as occurswith grease need not be a cause of concern and mounting is easy.

In accordance with the present invention, the sheet has electricalconductivity, unlike grease, and therefore using the sheet at a shieldedportion provides effects in relation also to EMC.

In accordance with the present invention, almost the same effects can beobtained regardless of whether the case surface is roughened or not.Since it suffices for surface roughening of the case to be of the lowestrank, a major reduction in cost can be expected. Further, experience isnot required, unlike the case with gels that need to be applied thinlyand evenly. Furthermore, a mounting structure that uses a sheet inaccordance with the present invention has a wide temperature range andcan be used in a temperature range from cryogenic temperatures, as areused in superconduction, to several hundred degrees centigrade.

Further, according to the present invention, a package and a circuitboard can be grounded simultaneously via a sheet. Moreover, surfacecontact is obtained by joint fastening using screws, and degradation ofmicrowave characteristics can be prevented. Further, even if there is astep at package- and circuit-board-bearing locations, severance of thesheet can be prevented by inclining the step.

Further, according to the present invention, malfunction due to releaseof radio waves and reception of radio waves, circuit vibration and abreak in a characteristic are eliminated. There are also effects inregard to ECM.

1-2. (canceled)
 3. A package mounting structure for mounting a packageon a case, the package internally incorporating at least one of ahigh-frequency transistor, MIC and MMIC used in the microwave tomillimeter-wave band, a base thereof being formed of metal and servingas ground, comprising: an electrically conductive sheet having thermalconductivity and exhibiting restorability, wherein in a case where stepsare present between a package-bearing location and circuit-board-bearinglocation on the case, step portions, which are among step portions onfour sides of the package-bearing location and through which microwavesdo not pass, are inclined, said sheet is laid on the case on theentirety of the package- and circuit-board-bearing locations, and eachof the package and circuit board and the sheet are fastened together andattached to the case by two or more screws.
 4. A package mountingstructure according to claim 3, wherein holes are provided in the sheetat portions through which the screws are passed, thereby resulting in asheetless state, and the package and sheet are fastened together by saidscrews, whereby the sheet is mounted on the case while the sheet ispressed by a pressing force of 10 N/cm² or greater
 5. A mountingstructure for mounting a superconducting circuit board in a metal case,comprising: an electrically conductive sheet having thermal conductivityand exhibiting restorability wherein said sheet is laid on the bottom ofa box-shaped metal case, a superconducting circuit board is placed onthe sheet, and said superconducting circuit board and sheet are fastenedtogether and attached to the metal case by two or more screws.
 6. Apackage mounting structure according to claim 5, wherein holes areprovided in the sheet at portions through which the screws are passed,thereby resulting in a sheetless state, and the package and sheet arefastened together by said screws, whereby the sheet is mounted on thecase while the sheet is pressed by a pressing force of 10 N/cm² orgreater
 7. A package mounting structure according to claim 6, whereinanother electrically conductive sheet exhibiting restorability andprovided with holes at portions through which screws are passed isplaced on peripheral top portions of the box-shaped metal case, a coverof the case is placed thereon, and the cover and sheet are fastenedtogether by screws.
 8. A package mounting structure according to claim7, wherein a recess smaller than the thickness of the sheet is formed ineither the peripheral top portion or the cover of said case. 9.(canceled)